Thunderstorm brings flight to violent end

(NYC06FA155)

The warmer temperatures of spring beckon many pilots back to the sky, but they also supply a key ingredient in the formation of one of aviation’s most violent killers—the thunderstorm. Wise pilots respect these convective beasts and give them a wide berth. But others choose to flirt rather than skirt, and this dangerous gamble can have catastrophic consequences.

On June 25, 2006, the pilot of a Piper PA-34 Seneca attempted to fly through a line of convective activity over Tafton, Pa. The extreme turbulence from a developing thunderstorm ripped the aircraft to pieces, killing the pilot and his two passengers. Witnesses said debris from the disintegrated airplane continued falling for up to 10 minutes after the encounter. Several large pieces of the aircraft were never found.

The 1,700-hour, instrument-rated pilot had taken off from Piedmont Triad International Airport in Greensboro, N.C., destined for Sanford Regional Airport in Sanford, Maine. Prior to departure, he received a weather briefing from the Raleigh Automated Flight Service Station. The pilot and the briefer discussed thunderstorm activity along the route of the flight, and the pilot subsequently filed an IFR flight plan.

The first two hours of the flight were uneventful. At about 12:30 p.m., over Pennsylvania, the pilot contacted Wilkes-Barre air traffic control at an altitude of 7,000 feet. Four minutes later, ATC informed him of an area of severe weather located six miles ahead of the airplane’s position. The controller asked if the pilot had weather radar available. The pilot replied that he had a portable GPS receiver with weather radar, and that the unit was displaying weather at his one o’clock position. Shortly after that, the pilot asked ATC if he could fly direct to the Kingston VOR. It was the pilot’s final radio call.

Radar data indicated that the airplane descended nearly 2,000 feet in the final 40 seconds of flight. The Seneca’s last radar target was observed at an altitude of 5,300 feet, which likely was the point at which the aircraft disintegrated. A witness near the accident site said he heard the sound of an engine “revving up and down.” This was followed by a “muffled pop,” then silence.

Witnesses reported seeing pieces of the aircraft falling from the sky. One stated that small debris fell for a period of five to 10 minutes. The NTSB determined that the accident resulted from the pilot’s inadvertent encounter with a thunderstorm, which led to a loss of aircraft control and subsequent in-flight breakup.

The destructive force of thunderstorms cannot be overstated. In addition to extremely heavy rain, they can contain strong wind shear, large hail, and severe turbulence, each of which can damage or destroy an aircraft. And these phenomena don’t just occur within the storm itself. According to the Aeronautical Information Manual, “the visible thunderstorm cloud is only a portion of a turbulent system whose updrafts and downdrafts often extend far beyond the visible storm cloud. Severe turbulence can be expected up to 20 miles from severe thunderstorms.”

On-board weather depiction equipment is a valuable safety tool for pilots, but we must appreciate the limitations of these devices. According to its manufacturer, the yoke-mounted GPS unit in the accident airplane received updated Nexrad weather radar images about every five minutes. Much can happen in five minutes when dealing with fast-moving, rapidly developing thunderstorms.

Moreover, radar images only depict areas of precipitation, not turbulence. Attempting to “thread the needle” between those brightly colored blotches on a moving map is apt to take a pilot into some violently churning air. In addition to staying 20 miles away from large convective cells, the AIM recommends completely circumnavigating any area that contains more than 50 percent thunderstorm coverage. If skirting the area isn’t practical, it’s best to land at a nearby airport and tie down. Have a cup of coffee and wait it out. Nature’s extreme fireworks are best enjoyed with the airplane safely on the ground.